Frequency deviation compensation scheme and frequency deviation compensation method

ABSTRACT

When a frequency deviation compensation amount is compensated for by use of frequency shift, a phase offset occurs between adjacent input blocks included in a plurality of input blocks as divided, with the result that an error occurs in a reconstructed bit sequence. A frequency deviation compensation system of the invention is characterized by comprising: a frequency deviation compensation means for compensating for a frequency deviation occurring in a signal by use of frequency shift; and a phase offset compensation means for compensating for a phase offset occurring, in the signal, due to the frequency shift.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 15/033,955 filed on May 3, 2016, which is aNational Stage Entry of international application PCT/JP2014/005376,filed Oct. 13, 2014, which claims the benefit of priority from JapanesePatent Application 2013-236643 filed on Nov. 15, 2013, the disclosuresof all of which are incorporated in their entirety by reference herein.

TECHNICAL FIELD

The present invention relates to a frequency deviation compensationscheme and a frequency deviation compensation method.

BACKGROUND ART

The widespread use of the Internet has led to a rapid increase intraffic volume for backbone communication systems. This has created adesire for realization of practical optical communication systemsoperating at ultra-high speed exceeding 100 Gbps. One technologyattracting attention to realize ultrafast optical communication systemsis the digital coherent scheme that combines an optical phase modulationscheme with a polarization multiplexing and demultiplexing technique.

PTL 1 and NPL 1 respectively disclose techniques to compensate for afrequency deviation in digital coherent receivers.

The digital coherent receiver described in NPL 1 uses local oscillationlight whose oscillating frequency can be controlled to compensate for afrequency deviation by controlling the oscillating frequency of thelocal oscillation light in the direction opposite to a frequencydeviation setting value. However, the configuration described in NPL 1requires an arrangement for controlling the oscillating frequency of thelocal oscillation light.

The digital coherent receiver according to PTL 1 compensates forwaveform distortion by conducting an overlap-type fast Fourier transform(FFT) and inverse FFT (IFFT). This digital coherent receiver hascircuitry which includes an input unit, an FFT input frame generationunit, an FFT processing unit, a characteristic multiplication unit, anIFFT processing unit, an IFFT output frame extraction unit, and anoutput unit. Assuming that the input data consists of 256 parallelsignals and the window size for FFT and IFFT is 1,024, the digitalcoherent receiver according to PTL 1 operates as follows.

The input unit buffers the input data (time domain: 256 samples),generates a frame consisting of 512 samples every two clocks, andoutputs the frame to the FFT input frame generation unit.

With respect to the inputted sample frames, the FFT input framegeneration unit generates a frame consisting of 1,024 samples bycombining the current 512-sample frame with the immediately preceding512-sample frame and outputs the generated frame to the FFT processingunit.

The FFT processing unit transforms the inputted frame intofrequency-domain data and outputs it to the characteristicmultiplication unit.

With respect to the inputted frequency-domain data, the characteristicmultiplication unit multiplies characteristic parameters for eachfrequency component (1,024 frequencies) and outputs the results to theIFFT processing unit. The characteristic parameters are inputted, forexample, from an external area.

The IFFT processing unit transforms the inputted frame into time-domaindata and outputs it to the IFFT output frame extraction unit. In frontand behind of this frame outputted from the IFFT processing unit,discontinuous points are included.

The IFFT output frame extraction unit discards 256 samples each, i.e., aquarter of the window size, from the front and rear of the inputtedframe. If discontinuous points are within the area discarded by the IFFToutput frame extraction unit, no discontinuous points are generated inthe output obtained by joining 512 samples that have not been discarded.The IFFT output frame extraction unit outputs the processed frame to theoutput unit.

The output unit takes out every 256 samples per one clock from aninputted frame (512 samples outputted every two clocks) and outputs themto the subsequent stage in the form of parallel signals.

The above-described digital coherent receiver according to PTL 1includes circuitry for performing the above-mentioned overlap-type FFTand IFFT to inhibit discontinuous points from occurring.

CITATION LIST Patent Literature

[PTL 1] Japanese Unexamined Patent Application Publication No. 2011-9956

Non Patent Literature

[NPL 1] Z. Tao et al., “Simple, Robust, and Wide-Range Frequency OffsetMonitor for Automatic Frequency Control in Digital Coherent Receivers”,2007 33rd European Conference and Exhibition of Optical Communication(ECOC2007)

SUMMARY OF INVENTION Technical Problem

In the case where the digital coherent receiver compensates for afrequency deviation of a received optical signal, the receiver maycompensate for the frequency deviation through a frequency-shiftingprocess. However, when the digital coherent receiver including theoverlap-type FFT and IFFT compensates for a frequency deviation througha frequency-shifting process, a phase offset will occur between one of aplurality of frames into which an input signal is divided and theimmediately preceding or following frame. This will result in an errorin the finally recovered bit string. In particular, there is even a riskof temporarily interrupting communications if the employed communicationmode is not differential coding.

The present invention has been designed in view of the above-describedproblems and has an object of providing a frequency deviationcompensation scheme and a frequency deviation compensation method thatprevent errors which may be caused by a phase offset, even when afrequency deviation is compensated for through a frequency-shiftingprocess.

Solution to Problem

A frequency deviation compensation scheme of the present inventionincludes a frequency deviation compensation means for compensating for,by frequency shifting, a frequency deviation caused to a signal; and aphase offset compensation means for compensating for a phase offsetcaused to the signal due to the frequency shifting.

Another frequency deviation compensation scheme of the present inventionincludes a compensation amount calculation means for adjusting an amountof frequency deviation compensation of a signal in such a way that aphase offset caused to the signal is a predetermined amount; and afrequency deviation compensation means for compensating for a frequencydeviation of the signal based on the adjusted amount of frequencydeviation compensation.

A frequency deviation compensation method of the present inventionincludes compensating for, by frequency shifting, a frequency deviationcaused to a signal; and compensating for a phase offset caused to thesignal due to the frequency shifting. [0021] Another frequency deviationcompensation method of the present invention includes adjusting anamount of frequency deviation compensation of a signal in such a waythat a phase offset caused to the signal is a predetermined amount; andcompensating for the frequency deviation of the signal based on theadjusted amount of frequency deviation compensation.

Advantageous Effects of Invention

According to the present invention, errors attributable to a phaseoffset are prevented even when a frequency-shifting process is used forcompensating for a frequency deviation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a digital coherent opticalreceiver according to a first exemplary embodiment.

FIG. 2 is a block diagram illustrating the digital signal processingunit 104 according to the first exemplary embodiment.

FIG. 3 is a block diagram illustrating the frequency deviation roughcompensation unit 202 according to the first exemplary embodiment.

FIG. 4 is another block diagram illustrating the frequency deviationrough compensation unit 202 according to the first exemplary embodiment.

FIG. 5 illustrates example operations of overlap FDE according to thefirst exemplary embodiment.

FIG. 6 is a block diagram illustrating the frequency deviation roughcompensation unit 202 under the scheme for roughly estimating afrequency deviation as described in NPL 1.

FIG. 7 is a block diagram illustrating the frequency deviationcompensation unit 206 according to the first exemplary embodiment.

FIG. 8 is a block diagram illustrating the frequency deviationestimation unit 301 according to the first exemplary embodiment.

FIG. 9 is another block diagram illustrating the frequency deviationrough compensation unit 202 according to the first exemplary embodiment.

FIG. 10 is another block diagram illustrating the frequency deviationrough compensation unit 202 according to the first exemplary embodiment.

FIG. 11 is a block diagram illustrating the frequency deviation roughcompensation unit 202 according to a second exemplary embodiment.

FIG. 12 is another block diagram illustrating the frequency deviationrough compensation unit 202 according to the second exemplaryembodiment.

FIG. 13 is a block diagram illustrating the frequency deviation roughcompensation unit 202 according to a third exemplary embodiment.

FIG. 14 is a block diagram illustrating the frequency deviation roughcompensation unit 202 according to a fourth exemplary embodiment.

FIG. 15 is a block diagram illustrating the frequency deviationcompensation unit 206 according to a fifth exemplary embodiment.

FIG. 16 is another block diagram illustrating the frequency deviationcompensation unit 206 according to the fifth exemplary embodiment.

FIG. 17 is another block diagram illustrating the frequency deviationcompensation unit 206 according to the fifth exemplary embodiment.

FIG. 18 is another block diagram illustrating the frequency deviationcompensation unit 206 according to the fifth exemplary embodiment.

DESCRIPTION OF EMBODIMENTS First Exemplary Embodiment

A first exemplary embodiment of the present invention will now bedescribed. To begin with, the digital coherent scheme is described. Onetechnology attracting attention to realize ultrafast opticalcommunication systems is the digital coherent scheme that combines anoptical phase modulation scheme with a polarization multiplexing anddemultiplexing technique.

The optical phase modulation scheme is a scheme for data modulationimposed on the phase of transmitted laser light, unlike the opticalintensity modulation scheme where data modulation is imposed on theoptical intensity of transmitted laser light. Some optical phasemodulation schemes are known, such as QPSK (Quadruple Phase ShiftKeying) and 16 QAM (16 Quadrature Amplitude Modulation) schemes.

According to the polarization multiplexing and demultiplexing technique,optical carrier waves are set in a single frequency band and twoindependent single-polarization optical signals whose polarizationstates are orthogonal to each other are polarization-multiplexed in anoptical transmitter. Then, in an optical receiver, these two independentsingle-polarization optical signals are separated from the receivedoptical signal. The polarization multiplexing and demultiplexingtechnique achieves a two-fold transmission speed.

FIG. 1 illustrates a block diagram of an optical receiver based on thedigital coherent scheme. The local oscillation light generation unit 100transmits local oscillation light in the same frequency band shared withthe received optical signal. Note that a frequency of optical signals onthe transmitter side and a frequency of local oscillation light on thereceiver side are predetermined by, for example, the administrator, whomakes settings of these frequencies to the respective light sources.

The received optical signal is inputted to 90 degrees hybrid 101 alongwith local oscillation light that is transmitted from the localoscillation light generation unit 100. The 90 degrees hybrid 101 outputsa total of eight optical signals to photo-electric conversion units102-1 to 102-4, two signals each, through coherence of the inputtedreceived light signal and the local oscillation light.

The photo-electric conversion units 102-1 to 102-4 each convert the twoinputted optical signals into electrical signals and outputs theelectrical signals to AD converters (ADCs; analog-to-digital converters)103-1 to 103-4, respectively.

The AD converters 103-1 to 103-4 convert the inputted analog electricalsignals into digital signals and outputs them to a digital signalprocessing unit 104. From the AD converters 103-1 to 103-4, four digitalsignals are outputted, which respectively correspond to the real partand imaginary part of a signal component (X polarization signal)parallel to the polarizing axis of the 90 degrees hybrid 101, and thereal part and imaginary part of a signal component (Y polarizationsignal) orthogonal to the polarizing axis of the 90 degrees hybrid 101.

The four digital signals outputted from the AD converters 103-1 to 103-4undergo demodulation process through the digital signal processing unit104, and then are recovered into bit strings in symbol identificationunits 105-1 and 105-2.

Now, the following provides detail descriptions of digital signalprocessing operations performed in the optical receiver based on thedigital coherent scheme. FIG. 2 illustrates a block diagram of thedigital signal processing unit 104.

An X polarization signal generation unit 200 generates an X polarizationsignal, which represents a complex number, from the digital signalsinputted from the ADCs 103-1 and 103-2, and then outputs the Xpolarization signal to a frequency deviation rough compensation unit202-1. On the other hand, a Y polarization signal generation unit 201generates a Y polarization signal, which represents a complex number,from the digital signals inputted from the ADCs 103-3 and 103-4, andthen outputs the Y polarization signal to a frequency deviation roughcompensation unit 202-2.

With regard to the inputted polarization signals, the frequencydeviation rough compensation units 202-1 and 202-2 compensate for adeviation between the center frequency of the received optical signaland the oscillating frequency of the local oscillation light with roughaccuracy. A deviation between the center frequency of a received opticalsignal and the oscillating frequency of local oscillation light ishereinafter referred to as an optical carrier frequency deviation.

A greater amount of optical carrier frequency deviation may sometimescause malfunction in a polarization demultiplexing unit 204 situated ina subsequent stage, depending on the type of the phase modulation schemeused for received optical signals or the optical signal-noise (SN)ratio. In addition, if a matched filter is placed in waveform distortioncompensation units 203-1 and 203-2 situated in a subsequent stage, adeviation between the received optical signal and the center frequencyof the matched filter will degrade signal quality. Note that thefrequency deviation rough compensation units 202-1 and 202-2 may beomitted if there is no such problems.

FIG. 3 illustrates a block diagram of the frequency deviation roughcompensation unit 202. A frequency deviation setting unit 401 sets afrequency deviation, and a phase compensation amount calculation unit402 calculates an amount of phase compensation based on the setfrequency deviation. An amount of phase compensation is calculated byobtaining the sum of products of a frequency deviation setting value andunit sampling times (inverse numbers of sampling rates for ADCs 103-1 to103-4).

The input signal inputted to the frequency deviation rough compensationunit 202 waits in a delay device 400 until an amount of phasecompensation is calculated. After the amount of phase compensation iscalculated, the input signal is subjected to frequency deviationcompensation through clockwise (opposite to the positive phasedirection) phase rotation by the calculated amount of phasecompensation.

Phase compensation can also be performed by shifting an optical spectrumin frequency direction in frequency domain. FIG. 4 illustrates a blockdiagram of the frequency deviation rough compensation unit 202, which isconfigured for this purpose. The frequency deviation rough compensationunit 202 illustrated in FIG. 4 performs phase compensation by shiftingan optical spectrum in the frequency direction in the frequency domain.This is called Frequency Domain Equalization (FDE). The FDE-based schemefor optical carrier frequency deviation compensation is effective inreducing a circuit size, owing to the simple process where data needonly be shifted in the frequency direction by the amount of frequencydeviation compensation, as well as owing to the capability tosimultaneously compensate for other linear distortions.

As illustrated in FIG. 5, the FDE-based frequency deviation roughcompensation unit 202 first divides an input signal into input blocks ofa predetermined length. An overlap addition unit 403 combines each ofthe input blocks with data of a predetermined length (overlap sizeN_(overlap)) in the latter part of the immediately preceding inputblock. As a result, FDE process blocks, each of which is in data lengthof the FFT/IFFT window size N_(FFT), are generated.

An FFT unit 404 performs a fast Fourier transform (FFT) on each of thegenerated FDE process blocks to transform it into a frequency-domainsignal. A frequency shifting unit 405 performs frequency shifting on thepost-fast Fourier transform FDE process block in the direction oppositeto the frequency deviation setting value. Any data on one of theboarders of an FDE process block deviating from the frequency range as aresult of the frequency shifting is deleted. On the other hand, zerosdepending on the amount of frequency shifting are inserted to theopposite boarder of the FDE process block.

An IFFT unit 406 performs an inverse fast Fourier transform (IFFT) onthe FDE process block to re-transform it into a time-domain signal, andoutputs the signal to an overlap deletion unit 407. The overlap deletionunit 407 deletes data of half the overlap size from the front and therear, respectively, of the FDE process block to generate resultingoutput data.

The overlap addition and deletion processes are performed in order toeliminate a computational distortion caused by the assumption in FFT andIFFT that a signal repeats periodically. The FDE involving theabove-described overlap addition and deletion processes is called anoverlap FDE scheme.

The local oscillation light generation unit 100, which is capable ofcontrolling oscillating frequencies, can also compensate for a frequencydeviation by controlling the oscillating frequency of the localoscillation light in the direction opposite to a frequency deviationsetting value. This scheme is disclosed in NPL 1 as described inBackground Art above. FIG. 6 illustrates a block diagram of thefrequency deviation rough compensation unit 202 used for compensatingfor a frequency deviation in accordance with the scheme described in NPL1.

In FIG. 6, a real part extraction unit 412 and an imaginary partextraction unit 413 extract the real part and the imaginary part,respectively, of an input signal. Next, a difference between products ofthe preceding and following two samples is calculated on each of theextracted real part and imaginary part, and then the signal goes througha low-pass filter 414 such as moving average. A frequency deviationcalculation unit 415 estimates a frequency deviation from the outputvalue of the low-pass filter 414.

Simulations demonstrate that an output value of the low-pass filter 414is proportional to a frequency deviation as far as the frequencydeviation is within a predetermined range. Accordingly, a frequencydeviation can be estimated from the output value of the low-pass filter414.

Now the digital signal processing unit 104 will be described again withreference to FIG. 2. Waveform distortion compensation units 203-1 and203-2 perform various compensation processes on the signal inputted fromthe frequency deviation rough compensation units 202-1 and 202-2 forimproving transmission quality, and then outputs the resulting receivedoptical signal to a polarization demultiplexing unit 204. Compensationprocesses performed by the waveform distortion compensation units 203-1and 203-2 for improving transmission quality include, for example,wavelength dispersion compensation, waveform shaping through a matchedfilter, and non-linear waveform distortion compensation.

The polarization demultiplexing unit 204 separates the inputted receivedoptical signal into two digital signals respectively corresponding totwo independent optical signals that underwent polarization multiplexingin the optical transmitter, and then individually outputs the separateddigital signals to resampling units 205-1 and 205-2. The polarizationdemultiplexing unit 204 uses an algorithm for polarizationdemultiplexing, such as Continuous Modulus Algorithm (CMA) or DecisionDecided Least Mean Square (DD-LMS).

Each of the resampling units 205-1 and 205-2 converts the inputteddigital signal to a signal oversampled by a factor of 1 with optimizedsampling timing, and outputs the resulting signal to a frequencydeviation compensation unit 206-1 or 206-2. The signals to be inputtedto the frequency deviation compensation units 206-1 and 206-2 must beoversampled by a factor of 1. In other words, the resampling units 205-1and 205-2 may be placed elsewhere before the frequency deviationcompensation units 206-1 and 206-2, such as a position preceding thepolarization demultiplexing unit 204.

The frequency deviation compensation units 206-1 and 206-2 completelycompensate the inputted signals for any optical carrier frequencydeviation which the frequency deviation rough compensation units 202-1and 202-2 may have failed to compensate for, and then output theresulting signals to phase deviation compensation units 207-1 and 207-2.FIG. 7 illustrates a block diagram of the frequency deviationcompensation units 206-1 and 206-2.

As illustrated in FIG. 7, the frequency deviation compensation units206-1 and 206-2 each include a delay device 300, a frequency deviationestimation unit 301, and a phase compensation amount calculation unit302. A signal inputted to either of the frequency deviation compensationunits 206-1 and 206-2 is bifurcated and inputted to the delay device 300and the frequency deviation estimation unit 301. The frequency deviationsetting unit 301 sets a frequency deviation using one part of thebifurcated input signal. FIG. 8 illustrates a block diagram of thefrequency deviation estimation unit 301. The frequency deviationestimation unit 301 sets a frequency deviation using M-th poweralgorithm or Viterbi algorithm. In order to use these algorithms,signals oversampled by a factor of 1 with optimized sampling timing needto be used. In addition, since signals oversampled by a factor of 1 areused, there is a limit imposed on a range of frequency deviations thatcan be compensated for.

The phase compensation amount calculation unit 302 calculates an amountof phase compensation based on the set frequency deviation, and thengives the calculated amount to the signal inputted to the delay device300. The signal inputted to the delay device 300 undergoes clockwise(opposite to the positive phase direction) phase rotation by the amountof phase compensation calculated by the phase compensation amountcalculation unit 302, thereby the frequency deviation is compensatedfor, and then the resulting signal is inputted to the phase deviationcompensation unit 207-1 or 207-2.

Note that the frequency deviation compensation unit 206 may beconfigured similarly to the frequency deviation rough compensation unit202 illustrated in FIG. 3 or FIG. 4. In this case, the frequencydeviation compensation unit 206 includes a frequency deviation settingunit instead of the frequency deviation estimation unit 301.

The phase deviation compensation units 207-1 and 207-2 compensate foroptical phase deviations of the inputted signals, and output theresulting signals to the symbol identification units 105-1 and 105 2.

The above-described digital coherent scheme that combines an opticalphase modulation scheme with a polarization multiplexing anddemultiplexing technique can realize a 100 Gbps ultrafast opticalcommunication system.

As described above, the frequency deviation rough compensation unit 202according to the present exemplary embodiment compensates for afrequency deviation by performing frequency shifting on data in thefrequency direction by an amount of frequency deviation compensation.However, if a frequency deviation is compensated for through afrequency-shifting process in the frequency domain in the frequencydeviation rough compensation unit 202 described with reference to FIGS.3, 4, and 6, a phase offset will occur between adjoining blocks to causean error in the recovered bit string. In particular, there is a risk oftemporarily interrupting communications if the employed communicationmode is not differential coding.

Thus, the present exemplary embodiment solves the above-describedproblem by providing the frequency deviation rough compensation unit 202which includes a phase offset compensation unit and a phase offsetamount calculation unit. A block diagram of the frequency deviationrough compensation unit 202 according to the present exemplaryembodiment is illustrated in FIG. 9. The frequency deviation roughcompensation unit 202 in FIG. 9 includes a phase offset compensationunit 408 and a phase offset amount calculation unit 409.

The frequency deviation setting unit 401 outputs a frequency deviationsetting value, as an amount of frequency deviation compensation, to thefrequency shifting unit 405 and to the phase offset amount calculationunit 409. Assuming here that the overlap FDE process oversamples asignal by a factor of 2, the amount of frequency deviation compensationΔf for frequency shifting can be expressed by Equation 1 with a symbolrate B, an FFT/IFFT window size N_(FFT), and an integer n. Note that theamount of frequency deviation compensation Δf and the amount of afrequency deviation are of opposite sign.

$\begin{matrix}{{\Delta \; f} = {\frac{2\; B}{N_{FFT}}n}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

In Equation 1, 2B represents a sampling rate w for the overlap FDEprocess; and n is an integer satisfying −N_(FFT)/2 n≤N_(FFT)/2.

Next, a phase offset Δφ occurring between the data at the end of dataoutputted through processing of the nth FDE process block and the dataat the beginning of data outputted through processing of the (n+1)th FDEprocess block can be calculated according to Equation 2.

$\begin{matrix}\begin{matrix}{{\Delta \; \varphi} = {{2{\pi\Delta}\; f\frac{N_{FFT} - {N_{overlap}/2}}{2B}} - {2{\pi\Delta}\; f\frac{N_{overlap}/2}{2B}}}} \\{= {2{\pi\Delta}\; f\frac{N_{FFT} - N_{overlap}}{2B}}} \\{= {2\pi \frac{2B}{N_{FFT}}n\frac{N_{FFT} - N_{overlap}}{2B}}} \\{= {2\pi \frac{N_{FFT} - N_{overlap}}{N_{FFT}}n}}\end{matrix} & {{Equation}\mspace{14mu} 2}\end{matrix}$

This equation is derived because the phase offset Δφ is obtained from adifference between the amount of phase rotation caused by the amount offrequency deviation compensation Δf in the nth FDE process block and theamount of phase rotation caused by the amount of frequency deviationcompensation Δf in the (n+1)th FDE process block.

A phase offset Δφ is generated every time the FDE process block numberis incremented (that is, every time the value n is incremented);therefore, the phase offset Δφ of the mth FDE process block is expressedby Equation 3.

$\begin{matrix}{{\Delta \; {\varphi \lbrack m\rbrack}} = {2\pi \frac{N_{FFT} - N_{overlap}}{N_{FFT}}{n\left( {m - 1} \right)}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

The phase offset amount calculation unit 409 calculates a phase offsetΔφ based on the circuit parameters having a frequency deviationcompensation amount inputted from the frequency deviation setting unit401, an FFT/IFFT window size, and an overlap size, and then outputs thecalculated phase offset to the phase offset compensation unit 408.

The phase offset compensation unit 408 compensates for a phase offsetcaused by frequency shifting, through inverse rotation by Δφ of thephase of data included in the FDE process block.

Note that, although the phase offset compensation unit 408 in thefrequency deviation rough compensation unit 202 of the present exemplaryembodiment performs phase rotation on frequency-domain data, the phaserotation may alternatively be performed on time-domain data. FIG. 10illustrates a block diagram of the frequency deviation roughcompensation unit 202, which is configured for this purpose.

As described above, the frequency deviation rough compensation unit 202of the present exemplary embodiment includes the phase offsetcompensation unit 408 and the phase offset amount calculation unit 409,which makes it possible to compensate for a phase offset caused byfrequency shifting. This provides the effect of preventing errors thatmay arise from a phase offset even if a frequency deviation iscompensated for through a frequency-shifting process during frequencydeviation compensation.

Note that the frequency deviation compensation units 206-1 and 206-2 maybe configured similarly to either of the frequency deviation roughcompensation units 202 illustrated in FIGS. 9 and 10. In this case, aphase offset caused by frequency shifting can also be compensated for inthe frequency deviation compensation unit 206.

Second Exemplary Embodiment

A second exemplary embodiment will now be described. Some descriptionsare omitted here for configurations similar to those in the firstexemplary embodiment. FIG. 11 illustrates a block diagram of thefrequency deviation rough compensation unit 202 according to the presentexemplary embodiment. The frequency deviation rough compensation unit202 in FIG. 11 includes a frequency shift amount calculation unit 410.

The frequency shift amount calculation unit 410 calculates a phaseoffset Δφ caused by frequency shifting process, based on the amount offrequency deviation Δf inputted from the frequency deviation settingunit 401 and in accordance with Equation 3. The frequency shift amountcalculation unit 410 further calculates an amount of frequency deviationΔf′ in such a way that the calculated phase offset Δφ is always aninteger multiple of 2π, and then notifies the frequency shifting unit405 of the calculated amount.

For example, if the FFT/IFFT window size is 1,024 and the overlap sizeis 256, the phase offset according to Equation 3 is represented byEquation 4.

$\begin{matrix}{{\Delta \; {\varphi \lbrack m\rbrack}} = \frac{3\pi \; {n\left( {m - 1} \right)}}{2}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

Accordingly, as far as n is a multiple of 4, the phase offset Δφ isalways an integer multiple of 2π irrespective of the value m.

The frequency shifting unit 405 performs frequency shifting process byusing the amount of frequency deviation compensation Δf′ inputted fromthe frequency shift amount calculation unit 410. The phase offset valueis now an integer multiple of 2π, which means the phase offset isequivalent to zero, and thus compensation for a phase offset isunnecessary. Any difference between Δf and Δf′ will be compensated forin the frequency deviation compensation units 206-1 and 206-2 situatedin a later stage.

While the present exemplary embodiment described above is configured toadjust the amount of frequency deviation compensation in such a way thatthe phase offset is an integer multiple of 2π, the frequency deviationrough compensation unit 202 illustrated in FIG. 12 further allows forreduction in a difference between Δf and Δf′. The frequency deviationrough compensation unit 202 in FIG. 12 includes a parameter control unit411.

The parameter control unit 411 determines a suitable FFT/IFFT windowsize based on the amount of frequency deviation compensation inputtedfrom the frequency deviation setting unit 401, and outputs thedetermined size to the FFT unit 404 and/or the IFFT unit 406. The FFTunit 404 and/or the IFFT unit 406 adjusts the FFT/IFFT window size foran input signal to the FFT/IFFT window size that has been inputted fromthe parameter control unit 411.

In addition, the parameter control unit 411 determines a suitableoverlap size based on the amount of frequency deviation compensationinputted from the frequency deviation setting unit 401, and outputs thedetermined size to the overlap addition unit 403. The overlap additionunit 403 adjusts the overlap size for an input signal to the overlapsize that has been inputted from the parameter control unit 411.

The frequency deviation rough compensation unit 202 in FIG. 12 canreduce a difference between Δf and Δf′ because of adjusting the FFT/IFFTwindow size and the overlap size based on an amount of frequencydeviation compensation.

The present exemplary embodiment makes it possible to compensate for aphase offset caused by frequency shifting. Consequently, there isprovided the effect of preventing errors that may arise from a phaseoffset even if a frequency deviation is compensated for through afrequency-shifting process during frequency deviation compensation.

Third Exemplary Embodiment

A third exemplary embodiment will now be described with reference to thedrawings. Some descriptions are omitted here for configurations similarto those in the first and second exemplary embodiments. FIG. 13illustrates a block diagram of the frequency deviation roughcompensation unit 202 according to the present exemplary embodiment. Asillustrated in FIG. 13, the frequency deviation rough compensation unit202 includes frequency deviation compensation means 405′ and phaseoffset compensation means 408′. Note that the frequency deviationcompensation means 405′ corresponds to the frequency shifting unit 405of the first and second exemplary embodiments, while the phase offsetcompensation means 408′ corresponds to the phase offset compensationunit 408 of the first exemplary embodiment.

The frequency deviation compensation means 405′ compensates for afrequency deviation of a frequency-domain signal by, for example,shifting the signal in the frequency direction. The phase offsetcompensation means 408′ compensates for a phase offset caused byshifting the signal in the frequency direction by, for example,inversely rotating the phase of the signal by the phase offset.

The frequency deviation rough compensation unit 202 according to thepresent exemplary embodiment includes the phase offset compensation unit408′, which makes it possible to compensate for a phase offset caused byshifting a signal in the frequency direction.

Fourth Exemplary Embodiment

A fourth exemplary embodiment will now be described with reference tothe drawings. Some descriptions are omitted here for configurationssimilar to those in the first to third exemplary embodiments. FIG. 14illustrates a block diagram of the frequency deviation roughcompensation unit 202 according to the present exemplary embodiment. Asillustrated in FIG. 14, the frequency deviation rough compensation unit202 includes frequency deviation compensation means 405′, frequencydeviation calculation means 401′, and compensation amount calculationmeans 410′. Note that the frequency deviation compensation means 405′corresponds to the frequency shifting unit 405 of the first and secondexemplary embodiments. Also note that the compensation amountcalculation means 410′ corresponds to the frequency shift amountcalculation unit 410 of the first and second exemplary embodiments.

The frequency deviation calculation means 401′ calculates an amount offrequency deviation compensation in a frequency-domain signal, and thenoutputs the calculated amount to the compensation amount calculationmeans 410′.

The compensation amount calculation means 410′ calculates a phase offsetΔφ caused by frequency shifting, based on the inputted amount offrequency deviation Δf, and then calculates an amount of frequencydeviation Δf′ in such a way that the phase offset Δφ is always aninteger multiple of 2π. The compensation amount calculation means 410′outputs the calculated amount of frequency deviation Δf′ to thefrequency deviation compensation means 405′.

The frequency deviation compensation means 405′ compensates for afrequency deviation of the signal based on the inputted amount offrequency deviation Δf′. Note that the frequency deviation compensationmeans 405′ compensates for a frequency deviation of the signal by, forexample, shifting the signal in the frequency direction.

The present exemplary embodiment makes it possible to compensate for aphase offset caused by shifting a signal in the frequency directionduring frequency deviation compensation.

Fifth Exemplary Embodiment

A fifth exemplary embodiment will now be described. The optical receiveraccording to the present exemplary embodiment is configured similarly tothe optical receiver in FIG. 1 and includes the digital signalprocessing unit 104 illustrated in FIG. 2. In the present exemplaryembodiment, the frequency deviation rough compensation units 202-1 and202-2 compensate for a frequency deviation by the amount of frequencydeviation compensation which has been set by the frequency deviationsetting unit 401. On the other hand, the frequency deviationcompensation units 206-1 and 206-2 compensate for a frequency deviationwhile dynamically changing an amount of frequency deviationcompensation.

In the digital coherent scheme, if an amount of frequency deviationcompensation is dynamically changed, a phase offset will occur betweenadjoining blocks to cause an error in the recovered bit string. Inparticular, there is a risk of temporarily interrupting communicationsif the employed communication mode is not differential coding.

Thus, in the present exemplary embodiment, the frequency deviationcompensation unit 206 includes a function to compensate for a phaseoffset caused by change in an amount of frequency deviationcompensation, thereby preventing errors that may arise from a phaseoffset even when the amount of frequency deviation compensation isdynamically changed. FIG. 15 illustrates a block diagram of thefrequency deviation compensation unit 206, which is configured for thispurpose.

With reference to FIG. 15, the overlap addition unit 503 functions inthe same manner as the overlap addition unit 403 in the frequencydeviation rough compensation unit 202 illustrated in FIG. 9. The FFTunit 504, the frequency shifting unit 505, the IFFT unit 506, theoverlap deletion unit 507, and the phase offset compensation unit 508function in the same manner as the FFT unit 404, the frequency shiftingunit 405, the IFFT unit 406, the overlap deletion unit 407, and thephase offset compensation unit 408, respectively.

The frequency deviation estimation unit 501 outputs a frequencydeviation estimated value, as an amount of frequency deviationcompensation, to the frequency shifting unit 505 and to the phase offsetamount calculation unit 509. Assuming that Δf_(n) represents the amountof frequency deviation compensation in the FDE process block n, Δf_(n+1)represents the amount of frequency deviation compensation in the FDEprocess block n+1, and Δf=Δf_(n+1)−Δf_(n) represents an amount of changein the amount of frequency deviation compensation, Δf is expressed byEquation 5 with a sampling rate fs, an FFT/IFFT window size N_(FFT), andan integer n.

$\begin{matrix}{{\Delta \; f} = {{{\Delta \; f_{n + 1}} - {\Delta \; f_{n}}} = {\frac{f_{s}}{N_{FFT}}n}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

In addition, a phase offset between the data at the end of an outputblock for the FDE process block n and the data at the beginning of anoutput block for the FDE process block n+1 can be calculated accordingto Equation 6. Equation 6 represents that the signal phase advances byΔf in the FDE process block n+1.

$\begin{matrix}{{\Delta \; \varphi} = {{2{\pi\Delta}\; f\frac{N_{overlap}/2}{f_{s}}} = {\pi \frac{N_{overlap}}{N_{FFT}}n}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

The phase offset amount calculation unit 509 calculates a phase offsetΔφ based on the circuit parameters having the inputted frequencydeviation compensation amount, the FFT/IFFT window size, and the overlapsize, and then outputs the calculated phase offset to the phase offsetcompensation unit 508.

The phase offset compensation unit 508 compensates for a phase offsetcaused by change in the amount of frequency deviation compensation, byinversely (clockwise) rotating the phase of data included in the FDEprocess block n+1 by the phase offset Δφ.

In the frequency deviation compensation unit 206 illustrated in FIG. 15,the phase offset compensation unit 508 performs phase rotation onfrequency-domain data; however, the phase offset compensation unit 508may also perform phase rotation on time-domain data. FIG. 16 illustratesa block diagram of the frequency deviation compensation unit 206, whichis configured for this purpose.

In addition, the frequency deviation compensation unit 206 may beconfigured as illustrated in FIG. 17. The frequency deviationcompensation unit 206 in FIG. 17 includes a frequency shift amountcalculation unit 510.

The frequency shift amount calculation unit 510 calculates, based on theamount of frequency deviation compensation inputted from the frequencydeviation estimation unit 501 and in accordance with Equation 2, a phaseoffset caused by change in the amount of frequency deviationcompensation, and approximates the phase offset to a value Δφ′ that isclosest to the phase offset value among integer multiples of 2π. Thefrequency shift amount calculation unit 510 calculates an amount ofchange Δf′ in the amount of frequency deviation compensationcorresponding to Δφ′ by using Equation 2, as well as calculating a newamount of frequency deviation compensation Δf_(n+1)′=f_(n)+Δf′ by usingEquation 1, and then outputs Δf_(n+1)′, an amount of frequency deviationcompensation, to the frequency shifting unit 505.

The frequency shifting unit 505 performs frequency shifting by using theinputted amount of frequency deviation compensation Δf_(n+1)′, on theother hand, compensation for a phase offset is unnecessary because thephase offset is an integer multiple of 2π, which means the phase offsetis equivalent to zero.

In addition, assuming that the FFT/IFFT window size is 1,024 and theoverlap size is 256, the phase offset is always an integer multiple ofπ/4. If n is a multiple of 8, the phase offset is an integer multiple of2π, and thus the phase offset is equivalent to zero. Thus, in accordancewith Equation 1, as far as the amount of frequency deviationcompensation Δf_(n) is limited beforehand to a product of a valueobtained by dividing the sampling rate by the FFT/IFFT window size and amultiple of 8, the phase offset is always an integer multiple of 2π,representing that no problem is caused by a phase offset.

While the present exemplary embodiment described above is configured toadjust the amount of frequency deviation compensation in such a way thata phase offset is an integer multiple of 2π, the frequency deviationcompensation unit 206 illustrated in FIG. 18 further allows forreduction in a difference between Δf_(n+1) and Δf_(n+1)′.

With reference to FIG. 18, the parameter control unit 511 determines asuitable FFT/IFFT window size based on the amount of frequency deviationcompensation inputted from the frequency deviation estimation unit 501,and outputs the determined size to the FFT unit 504 and/or the IFFT unit506. The FFT unit 504 and/or the IFFT unit 506 adjusts the FFT/IFFTwindow size for an input signal to the FFT/IFFT window size that hasbeen inputted from the parameter control unit 511.

In addition, the parameter control unit 511 determines a suitableoverlap size based on the amount of frequency deviation compensationinputted from the frequency deviation estimation unit 501, and outputsthe determined size to the overlap addition unit 503. The overlapaddition unit 503 adjusts the overlap size for an input signal to theoverlap size that has been inputted from the parameter control unit 511.

The frequency deviation compensation unit 206 in FIG. 18 can reduce adifference between Δf_(n+1) and Δf_(n+1)′ because of adjusting theFFT/IFFT window size and the overlap size based on an amount offrequency deviation compensation.

In the present exemplary embodiment, the frequency deviation roughcompensation units 202-1 and 202-2 are configured similarly to any oneillustrated in FIGS. 9 to 12, which makes it possible to compensate fora phase offset caused by frequency shifting. In addition, the frequencydeviation compensation units 206-1 and 206-2 have a configurationillustrated in any one of FIGS. 15 to 18, which makes it possible tocompensate for a phase offset caused when the amount of frequencydeviation compensation is dynamically changed during frequency deviationcompensation.

Consequently, the present exemplary embodiment provides the effect ofcompensating for a frequency deviation by performing afrequency-shifting process during frequency deviation roughcompensation, as well as providing the effect of preventing errors thatmay arise from a phase offset even when the amount of frequencydeviation compensation is dynamically changed during frequency deviationcompensation.

Alternatively, the frequency deviation rough compensation units 202-1and 202-2 may compensate for a frequency deviation by dynamicallychanging the amount of frequency deviation compensation, while thefrequency deviation compensation units 206-1 and 206-2 may compensatefor a frequency deviation by the amount of frequency deviationcompensation set by the frequency deviation setting unit. In this case,the frequency deviation compensation units 206-1 and 206-2 areconfigured similarly to any one illustrated in FIGS. 9 to 12, while thefrequency deviation rough compensation units 202-1 and 202-2 areconfigured similarly to any one illustrated in FIGS. 15 to 18.

Sixth Exemplary Embodiment

A sixth exemplary embodiment will now be described. According to thepresent exemplary embodiment, a computer, central processing unit (CPU),micro-processing unit (MPU), or the like for an optical receiverexecutes the software (program) that implements functions of theabove-described individual exemplary embodiments. The optical receiverobtains the software (program) that implements functions of theabove-described individual exemplary embodiments via any of variousstorage media such as CD-R (Compact Disc Recordable) or via a network. Aprogram obtained by the optical receiver or a storage medium storing theprogram is part of the present invention. Note that the software(program) may be stored, for example, in advance in a predeterminedstorage unit included in the optical receiver.

The computer, CPU, MPU, or the like for the optical receiver reads out aprogram code from the obtained software (program) and executes it.Accordingly, the optical receiver performs the same processes as thosefor an optical receiver according to the above-described individualexemplary embodiments.

The above-described exemplary embodiments represent processes performedafter an optical signal is converted to an electrical signal, and areapplicable to any optical modulation scheme that can be applied tooptical transmissions (optical communications).

The present invention is not limited to the above exemplary embodimentsand includes design changes and the like that do not depart from thegist of the present invention. The whole or part of the above exemplaryembodiments can be described as, but is not limited to, the followingsupplementary notes.

[Supplementary Note 1]

A frequency deviation compensation scheme including:

a frequency deviation compensation means for compensating for, byfrequency shifting, a frequency deviation caused to a signal; and

a phase offset compensation means for compensating for a phase offsetcaused to the signal due to the frequency shifting.

[Supplementary Note 2]

The frequency deviation compensation scheme according to SupplementaryNote 1,

wherein the phase offset compensation means compensates for the phaseoffset by inversely rotating the phase of the signal by the phaseoffset.

[Supplementary Note 3]

The frequency deviation compensation scheme according to SupplementaryNote 1 or 2, further including:

a phase offset calculation means for calculating a phase offset causedto the signal due to the frequency shifting,

wherein the phase offset compensation means compensates for the phaseoffset of the signal based on the calculated phase offset.

[Supplementary Note 4]

The frequency deviation compensation scheme according to SupplementaryNote 3, further including:

a frequency deviation calculation unit for calculating an amount offrequency deviation compensation in the signal,

wherein the frequency deviation compensation means compensates for thefrequency deviation of the signal based on the calculated amount offrequency deviation compensation, and

wherein the phase offset calculation means calculates the phase offsetbased on the calculated amount of frequency deviation compensation.

[Supplementary Note 5]

The frequency deviation compensation scheme according to SupplementaryNote 4,

wherein the frequency deviation calculation unit calculates an amount offrequency deviation compensation for each of a plurality of blocks intowhich the signal is divided,

wherein the frequency deviation compensation means compensates for afrequency deviation for each of the plurality of blocks, based on thecalculated amount of frequency deviation compensation, and

wherein the phase offset compensation means compensates for the phaseoffset which arises from a difference between an amount of phaserotation corresponding to an amount of frequency deviation compensationof one of the blocks and an amount of phase rotation corresponding to anamount of frequency deviation compensation of another block adjacent tothe block.

[Supplementary Note 6]

The frequency deviation compensation scheme according to any one ofSupplementary Notes 3 to 5,

wherein the phase offset calculation means calculates the phase offsetbased on the amount of frequency deviation compensation, an FFT/IFFTwindow size, and an overlap size.

[Supplementary Note 7]

The frequency deviation compensation scheme according to any one ofSupplementary Notes 1 to 6,

wherein the phase offset compensation means compensates for the phaseoffset by inversely rotating the phase of the signal in a frequencydomain by the phase offset.

[Supplementary Note 8]

The frequency deviation compensation scheme according to any one ofSupplementary Notes 1 to 6,

wherein the phase offset compensation means compensates for the phaseoffset by inversely rotating the phase of the signal in a time domain bythe phase offset.

[Supplementary Note 9]

A frequency deviation compensation scheme including:

a compensation amount calculation means for adjusting an amount offrequency deviation compensation of a signal in such a way that a phaseoffset caused to the signal is a predetermined amount when a frequencydeviation of the signal is compensated for by frequency shifting; and

a frequency deviation compensation means for compensating for thefrequency deviation of the signal based on the adjusted amount ofcompensation.

[Supplementary Note 10]

The frequency deviation compensation scheme according to SupplementaryNote 9, wherein the predetermined amount is an integer multiple of 2π.

[Supplementary Note 11]

The frequency deviation compensation scheme according to SupplementaryNote 9 or 10, further including:

a parameter control means for adjusting at least one of an FFT/IFFTwindow size and an overlap size in such a way that a phase offset causedto the signal is a predetermined amount.

[Supplementary Note 12]

The frequency deviation compensation scheme according to any one ofSupplementary Notes 9 to 11, further including:

a frequency deviation calculation means for calculating a first amountof frequency deviation compensation as the amount of frequency deviationcompensation,

wherein the parameter control means:

-   -   adjusts an FFT/IFFT window size and an overlap size in such a        way that an FFT/IFFT window size is a positive number multiple        of an overlap size; and    -   sets, as a second amount of frequency deviation compensation, a        value closest to the first amount of frequency deviation        compensation among values obtained by dividing a sampling rate        by an FFT/IFFT window size and multiplying the result by an        integer multiple of twice the positive number, and

wherein the frequency deviation compensation means compensates for thefrequency deviation of the signal by shifting the signal in thefrequency direction based on the second amount of frequency deviationcompensation.

[Supplementary Note 13]

A frequency deviation compensation method including:

compensating for, by frequency shifting, a frequency deviation caused toa signal; and

compensating for a phase offset caused to the signal due to thefrequency shifting.

[Supplementary Note 14]

The frequency deviation compensation method according to SupplementaryNote 13, including:

compensating for the phase offset by inversely rotating the phase of thesignal by the calculated phase offset.

[Supplementary Note 15]

The frequency deviation compensation method according to SupplementaryNote 13 or 14, including:

calculating a phase offset caused to the signal due to the frequencyshifting; and

compensating for the phase offset of the signal based on the calculatedphase offset.

[Supplementary Note 16]

The frequency deviation compensation method according to any one ofSupplementary Notes 13 to 15, including:

calculating an amount of frequency deviation compensation in the signal;

compensating for the frequency deviation of the signal based on thecalculated amount of frequency deviation compensation; and

calculating the phase offset based on the calculated amount of frequencydeviation compensation.

[Supplementary Note 17]

The frequency deviation compensation method according to SupplementaryNote 16, including:

calculating an amount of frequency deviation compensation for each of aplurality of blocks into which the signal is divided;

compensating for a frequency deviation for each of the plurality ofblocks based on the calculated amount of frequency deviationcompensation; and

compensating for the phase offset which arises from a difference betweenan amount of phase rotation corresponding to an amount of frequencydeviation compensation of one of the blocks and an amount of phaserotation corresponding to an amount of frequency deviation compensationof another block adjacent to the block.

[Supplementary Note 18]

The frequency deviation compensation method according to any one ofSupplementary Notes 13 to 17, including:

calculating the phase offset based on the amount of frequency deviationcompensation, an FFT/IFFT window size, and an overlap size.

[Supplementary Note 19]

The frequency deviation compensation method according to any one ofSupplementary Notes 13 to 18, including:

compensating for the phase offset by inversely rotating the phase of thesignal in a frequency domain by the phase offset.

[Supplementary Note 20]

The frequency deviation compensation method according to any one ofSupplementary Notes 13 to 18, including:

compensating for the phase offset by inversely rotating the phase of thesignal in a time domain by the phase offset.

[Supplementary Note 21]

A frequency deviation compensation method including:

adjusting an amount of frequency deviation compensation of a signal insuch a way that a phase offset caused to the signal is a predeterminedamount when a frequency deviation of the signal is compensated for byfrequency shifting; and

compensating for the frequency deviation of the signal based on theadjusted amount of frequency deviation compensation.

[Supplementary Note 22]

The frequency deviation compensation method according to SupplementaryNote 21, wherein the predetermined amount is an integer multiple of 2π.

[Supplementary Note 23]

The frequency deviation compensation method according to SupplementaryNote 21 or 22, including:

adjusting at least one of an FFT/IFFT window size and an overlap size insuch a way that the phase offset is a predetermined amount.

[Supplementary Note 24]

The frequency deviation compensation method according to any one ofSupplementary Notes 21 to 23, including:

calculating a first amount of frequency deviation compensation as theamount of frequency deviation compensation;

adjusting an FFT/IFFT window size and an overlap size in such a way thatan FFT/IFFT window size is a positive number multiple of an overlapsize;

setting, as a second amount of frequency deviation compensation, a valueclosest to the first amount of frequency deviation compensation amongvalues obtained by dividing a sampling rate by an FFT/IFFT window sizeand multiplying the result by an integer multiple of twice the positivenumber; and

compensating for the frequency deviation of the signal by shifting thesignal in the frequency direction based on the second amount offrequency deviation compensation.

[Supplementary Note 25]

A program causing a computer to execute the processes of:

compensating for, by frequency shifting, a frequency deviation caused toa signal; and

compensating for a phase offset caused to the signal due to thefrequency shifting.

[Supplementary Note 26]

The program according to Supplementary Note 25, including the processof:

compensating for the phase offset by inversely rotating the phase of thesignal by the calculated phase offset.

[Supplementary Note 27]

The program according to Supplementary Note 25 or 26, including theprocesses of:

calculating a phase offset caused to the signal due to the frequencyshifting; and

compensating for the phase offset of the signal based on the calculatedphase offset.

[Supplementary Note 28]

The program according to any one of Supplementary Notes 25 to 27,including the processes of:

calculating an amount of frequency deviation compensation of the signal;

compensating for the frequency deviation of the signal based on thecalculated amount of frequency deviation compensation; and

calculating the phase offset based on the calculated amount of frequencydeviation compensation.

[Supplementary Note 29]

The program according to any one of Supplementary Notes 25 to 28,including the processes of:

calculating an amount of frequency deviation compensation for each of aplurality of blocks into which the signal is divided;

compensating for a frequency deviation of each of the plurality ofblocks based on the calculated amount of frequency deviationcompensation; and

compensating for the phase offset which arises from a difference betweenan amount of phase rotation corresponding to an amount of frequencydeviation compensation of one of the blocks and an amount of phaserotation corresponding to an amount of frequency deviation compensationof another block adjacent to the block.

[Supplementary Note 30]

The program according to any one of Supplementary Notes 23 to 29,including the process of:

calculating the phase offset based on the amount of frequency deviationcompensation, an FFT/IFFT window size, and an overlap size.

[Supplementary Note 31]

The program according to any one of Supplementary Notes 23 to 30,including the process of:

compensating for the phase offset by inversely rotating the phase of thesignal in a frequency domain by the phase offset.

[Supplementary Note 32]

The program according to any one of Supplementary Notes 23 to 28,including the process of:

compensating for the phase offset by inversely rotating the phase of thesignal in a time domain by the phase offset.

[Supplementary Note 33]

A program causing a computer to execute the processes of:

adjusting an amount of frequency deviation compensation of a signal insuch a way that a phase offset caused to the signal is a predeterminedamount when a frequency deviation of the signal is compensated for byfrequency shifting; and

compensating for the frequency deviation of the signal based on theadjusted amount of frequency deviation compensation.

[Supplementary Note 34]

The program according to Supplementary Note 33, wherein thepredetermined amount is an integer multiple of 2π.

[Supplementary Note 35]

The program according to Supplementary Note 33 or 34, including theprocess of:

adjusting at least one of an FFT/IFFT window size and an overlap size insuch a way that the phase offset is a predetermined amount.

[Supplementary Note 36]

The program according to any one of Supplementary Notes 33 to 35,including the processes of:

calculating a first amount of frequency deviation compensation as theamount of frequency deviation compensation;

adjusting an FFT/IFFT window size and an overlap size in such a way thatan FFT/IFFT window size is a positive number multiple of an overlapsize;

setting, as a second amount of frequency deviation compensation, a valueclosest to the first amount of frequency deviation compensation amongvalues obtained by dividing a sampling rate by an FFT/IFFT window sizeand multiplying the result by an integer multiple of twice the positivenumber; and

compensating for the frequency deviation of the signal by shifting thesignal in the frequency direction based on the second amount offrequency deviation compensation.

The present application claims priority based on Japanese PatentApplication No. 2013-236643 filed on Nov. 15, 2013, the entiredisclosure of which is incorporated herein.

INDUSTRIAL APPLICABILITY

The present invention can be applied to any optical modulation schemethat is applied to optical communications.

REFERENCE SIGNS LIST

-   100 Local oscillation light generation unit-   101 90 degrees hybrid-   102-1, 102-2, 102-3, 102-4 Photo-electric conversion unit-   103-1, 103-2, 103-3, 103-4 ADC-   104 Digital signal processing unit-   105-1, 105-2 Symbol identification unit-   200 X polarization signal generation unit-   201 Y polarization signal generation unit-   202-1, 202-2 Frequency deviation rough compensation unit-   203-1, 203-2 Waveform distortion compensation unit-   204 Polarization demultiplexing unit-   205-1, 205-2 Resampling unit-   206-1, 206-2 Frequency deviation compensation unit-   207-1, 207-2 Phase deviation compensation unit-   300 Delay device-   301 Frequency deviation estimation unit-   302 Phase compensation amount calculation unit-   400 Delay device-   401 Frequency deviation setting unit-   401′ Frequency deviation calculation means-   402 Phase compensation amount calculation unit-   403, 503 Overlap addition unit-   404, 504 FFT unit-   405, 505 Frequency shifting unit-   405′ Frequency deviation compensation means-   406, 506 IFFT unit-   407, 507 Overlap deletion unit-   408, 508 Phase offset compensation unit-   408′ Phase offset compensation means-   409, 509 Phase offset amount calculation unit-   410, 510 Frequency shift amount calculation unit-   410′ Compensation amount calculation means-   411, 511 Parameter control unit-   412 Real part extraction unit-   413 Imaginary part extraction unit-   414 Low-pass filter-   415 Frequency deviation calculation unit-   501 Frequency deviation estimation unit

1. A digital coherent optical receiver comprising: a local oscillatorconfigured to output a local oscillation light; an optical hybridconfigured to receive an input optical signal by interference with thelocal oscillation light; a photo-electric convertor configured toconvert the received optical signal into an electric signal; an analogto digital (AD) converter configured to convert the electric signal intoa digital signal; and a digital signal processor configured to processthe digital signal, wherein the digital signal processor comprises: afrequency deviation compensator configured to compensate a frequencydeviation of the input optical signal by frequency shift process to thedigital signal; and a phase offset compensator configured to compensatea phase offset of the deviation compensated digital signal caused by thefrequency shift process.
 2. The digital coherent optical receiveraccording to claim 1, wherein the phase offset compensator is furtherconfigured to compensate the phase offset by inversely rotating a phaseof the digital signal.
 3. The digital coherent optical receiveraccording to claim 2, further comprising a phase offset calculatorconfigured to calculate a phase offset amount based on a frequencydeviation compensation amount compensated by the frequency deviationcompensator, an FFT/IFFT window size, and an overlap size.
 4. Thedigital coherent optical receiver according to claim 2, furthercomprising a phase offset calculator configured to calculate a phaseoffset amount, wherein the phase offset compensator is furtherconfigured to compensate the phase offset based on the calculated phaseoffset amount.
 5. The digital coherent optical receiver according toclaim 2, further comprising a frequency deviation estimator configuredto estimate the frequency deviation, wherein when a frequency deviationcompensation amount compensated by the frequency deviation compensatoris dynamically changed, the frequency deviation estimator is furtherconfigured to output a frequency deviation estimated value to thefrequency deviation compensator and the phase offset compensator.
 6. Thedigital coherent optical receiver according to claim 1, furthercomprising a phase offset calculator configured to calculate a phaseoffset amount, wherein the phase offset compensator is furtherconfigured to compensate the phase offset based on the calculated phaseoffset amount.
 7. The digital coherent optical receiver according toclaim 6, further comprising a frequency deviation calculator configuredto calculate a frequency deviation compensation amount, wherein thefrequency deviation compensator is further configured to compensate thefrequency deviation based on the calculated frequency deviationcompensation amount, and wherein the phase offset calculator is furtherconfigured to calculate the phase offset amount based on the calculatedfrequency deviation compensation amount.
 8. The digital coherent opticalreceiver according to claim 6, wherein the phase offset calculator isfurther configured to calculate the phase offset amount based on afrequency deviation compensation amount compensated by the frequencydeviation compensator, an FFT/IFFT window size, and an overlap size. 9.The digital coherent optical receiver according to claim 6, furthercomprising a frequency deviation estimator configured to estimate thefrequency deviation, wherein when a frequency deviation compensationamount compensated by the frequency deviation compensator is dynamicallychanged, the frequency deviation estimator is further configured tooutput a frequency deviation estimated value to the frequency deviationcompensator and the phase offset compensator.
 10. The digital coherentoptical receiver according to claim 1, further comprising a frequencydeviation estimator configured to estimate the frequency deviation,wherein when a frequency deviation compensation amount compensated bythe frequency deviation compensator is dynamically changed, thefrequency deviation estimator is further configured to output afrequency deviation estimated value to the frequency deviationcompensator and the phase offset compensator.
 11. The digital coherentoptical receiver according to claim 1, wherein the frequency deviationcompensator is further configured to compensate the frequency deviationupstream of a distortion compensator.
 12. The digital coherent opticalreceiver according to claim 1, wherein the frequency deviationcompensator is further configured to compensate the frequency deviationupstream of a polarization demultiplexer.
 13. A digital coherent opticalreceiver comprising: a local oscillator configured to output a localoscillation light; an optical hybrid configured to receive an inputoptical signal by interference with the local oscillation light; aphoto-electric convertor configured to convert the received opticalsignal into an electric signal; an analog to digital (AD) converterconfigured to convert the electric signal into a digital signal; and adigital signal processor configured to process the digital signal,wherein the digital signal processor comprises: a compensation amountcalculator configured to identify a frequency deviation compensationamount corresponding to a predetermined phase offset amount of thedigital signal; and a frequency deviation compensator configured tocompensate, based on the frequency deviation compensation amount, afrequency deviation of the input optical signal by frequency shiftprocess to the digital signal.
 14. The digital coherent optical receiveraccording to claim 13, wherein the predetermined phase offset amount isan integer multiple of 2π.
 15. The digital coherent optical receiveraccording to claim 14, further comprising a parameter controllerconfigured to adjust at least one of an FFT/IFFT window size and anoverlap size, the adjusted FFT/IFFT window size and the overlap sizecorresponding to the predetermined phase offset amount.
 16. The digitalcoherent optical receiver according to claim 13, wherein thepredetermined phase offset amount is equivalent to zero.
 17. The digitalcoherent optical receiver according to claim 13, further comprising aparameter controller configured to adjust at least one of an FFT/IFFTwindow size and an overlap size, the adjusted FFT/IFFT window size andthe overlap size corresponding to the predetermined phase offset amount.18. The digital coherent optical receiver according to claim 13, furthercomprising a frequency deviation estimator configured to estimate thefrequency deviation, wherein when the frequency deviation compensationamount compensated by the frequency deviation compensator is dynamicallychanged, the frequency deviation estimator is further configured tooutput a frequency deviation estimated value to the compensation amountcalculator.
 19. The digital coherent optical receiver according to claim13, wherein the frequency deviation compensator is further configured tocompensate the frequency deviation upstream of a distortion compensator.20. The digital coherent optical receiver according to claim 13, whereinthe frequency deviation compensator is further configured to compensatethe frequency deviation upstream of a polarization demultiplexer.